1. Field of the Invention
The present invention relates to a pneumatic tire which can discharge a static electricity generated in a vehicle body and a tire, to a road surface.
2. Description of the Related Art
Recently, there have been proposed a pneumatic tire including a tread rubber blended with silica at a high ratio in order to reduce the rolling resistance that largely affects the fuel consumption of a vehicle and/or to increase braking performance (WET braking performance) on a wet road surface. However, compared to a tread rubber blended with carbon black at a high ratio, the electric resistance of such tread rubber is high, and accordingly, static charge generated on a vehicle body or the tire is prevented from being released to the road surface. As a result, problems like radio noises tend to occur.
Accordingly, there has been developed a pneumatic tire structured such that a conductive portion constructed by a conductive rubber blended with a carbon black or the like is buried in a tread rubber constructed by a nonconductive rubber blended with a silica or the like, whereby an electrical conduction performance can be achieved. For example, in a pneumatic tire described in Japanese Unexamined Patent Publication No. 2003-326614 and Japanese Unexamined Patent Publication No. 2010-115935, a tread rubber formed by a nonconductive rubber is provided with a conductive portion which extends in a tire radial direction from a ground-contacting surface so as to reach a belt layer, thereby forming a conduction route for discharging a static electricity. However, in that tire, since the conductive route is formed via the belt layer, it can not correspond to a case that a topping rubber of the belt layer is formed by the nonconductive rubber.
On the other hand, in a pneumatic tire described in Japanese Unexamined Patent Publication No. 2009-126291, a tread rubber formed by a nonconductive rubber is provided with a conductive portion which extends to an inner side in a tire radial direction from a ground-contacting surface, extends in a tire width direction between a cap portion and a base portion, and is connected to a side wall rubber or a topping rubber of a carcass layer. However, in the conductive portion which is formed as an L-shaped cross section (refer to FIG. 13), since it is formed such a shape as to be biased to one side in the tire width direction, it has been found that uniformity in a lateral direction of the tire is deteriorated.
Accordingly, there can be thought that the conductive portion is formed as an inverse T-shaped form, and the conductive portion is extended to both sides in the tire width direction between the cap portion and the base portion, for improving the uniformity, however, in the case of structuring so, a strain of a shoulder region becomes large in both sides in the tire width direction, and a rolling resistance tends to be increased. On the contrary, in the case that the conductive portion is kept away from the ground-contacting surface in the shoulder regions in both sides, a strain can be lightened, however, since the shoulder region tends to wear by priority, an irregular wear resistance tends to be lowered.